We propose to conduct a longitudinal observational study of the neuropsychiatric sequelea of HIV infection in male homosexuals and heterosexual former intravenous drug abusers. Utilizing a longitudinal repeated measures design, we will evaluate 60 seropositive heterosexual intravenous drug abusers on methadone maintenance and 60 seropositive male homosexuals at six month intervals over a three year period. We will also follow 60 seronegative heterosexual intravenous drug abusers on methadone maintenance and 60 seronegative homosexual males as controls. Subjects will be drawn from two existing cohorts at San Francisco General Hospital (SFGH). Outcome measures will emphasize psychiatric symptomatology, medical and immune function analyses and neuropsychological and neurophysiological parameters. Data will be collected in each of these areas at regular intervals over the three year study period. Specific aims of the proposed study are as follows: 1. To estimate the incidence and prevalence of neuropsychological and neurophysiological dysfunction in HIV antibody positive homosexual males and former intravenous heroin users, and to determine whether intergroup differences exist. 2. To describe the natural history of neuropsychological and neurophysiological dysfunction over a three-year follow-up period. 3. To examine the interrelationships among neuropsychological, neurophysiological, psychiatric and immune function variables. 4. To determine whether the occurrence of neuropsychological and neurophysiological dysfunction may be used as a predictor of the subsequent development of particular disease states associated with diagnoses of AIDS and/or ARC. 5. To facilitate the development of an effective screening instrument for neuropsychiatric dysfunction in the populations studied.